Nowadays, ultraviolet laser lights, which have short wavelengths and high temporal coherence, are used in various fields. For example, ultraviolet laser light are used in exposure apparatus in semiconductor-device and in surgical and laser treatment apparatus for such operations as performed in surgery, ophthalmology and dentistry. Ultraviolet laser light are used also in various measuring instruments and analyzers and in processing apparatus. Ultraviolet light sources used in such apparatus include KrF excimer laser, which has an oscillation wavelength at λ=248 nm, and ArF excimer laser, which has an oscillation wavelength at λ=193 nm. Although these excimer lasers are already comprised as light sources in various apparatus, they have problems of troublesome maintenance and high running costs because these lasers use fluorine gas as an operational medium besides their being expensive and large in size.
Therefore, a study has been eagerly conducted for an all solid state ultraviolet light source that comprises as a signal light source a solid state laser oscillating in an infrared-to-visible range, that amplifies the light output from the solid state laser by a fiber optical amplifier, and that converts the amplified light into ultraviolet light having a predetermined wavelength as an output by a wavelength-converting optical system. As such an all solid state ultraviolet light source, a proposed ultraviolet light source comprises as a signal light source a DFB semiconductor laser oscillating stably in a band at wavelength λ=1.55 μm, amplifies the light output from the semiconductor laser to a desired light intensity by an erbium-doped fiber amplifier (hereinafter abbreviated to “EDFA”), and converts the amplified light to ultraviolet light having a wavelength λ=193 nm, which is the eighth harmonic, as an output by a wavelength-converting optical system, which comprises a crystal for wavelength conversion (refer to, for example, Japanese Laid-Open Patent Publication No. 2000-200747 and No. 2001-353176).
As EDFAs used in such an all solid state ultraviolet light source, known are a single-clad EDFA, which excites an erbium-doped single-clad and single mode fiber (EDF) by a semiconductor laser of single mode oscillation, and a double-clad EDFA, which excites a double-clad EDF by a semiconductor laser of multi-mode oscillation (refer to, for example, Japanese Laid-Open Patent Publication No. 2000-200747 and No. 2001-353176).
Yet, prior-art EDFAs such as those mentioned above have the following problems. First of all, for a single-clad EDFA, the optical power of the single mode oscillation semiconductor laser that excites the EDFA is limited within a range of hundreds of mW, which is comparatively low, so signal light at a low repetition frequency of a few kHz and at a low duty must be used to generate a peak power higher than 10 kW (with a pulse width of up to 1 ns). The light output from the EDFA in this case, even if a high peak power is gained, achieves only a relatively low power of about 100 mW at the most as an average power. This is a problem indicating that the original amplifying ability of the fiber optical amplifier is not utilized fully.
Also, in a fiber-optical amplifier like that described above, it is a common practice to arrange a plurality of EDFAs in series connection in a multiple stage construction to achieve a predetermined peak power. However the ASE lights generated in the EDFA are added as DC noises to the signal-less parts between adjacent signal pulses, which leads to a significant reduction in the signal-to-noise ratio of the output signal. Therefore, as an arrangement for removing DC noise components in a multi-staged EDFA, an electro-optic modulating element or an acousto-optic modulating element is provided between adjacent stages, and each modulating element is controlled to synchronize with a signal light source to supply signal light at a high signal-to-noise ratio to a rear stage EDFA. However, the construction of the EDFA becomes complicated, and the manufacturing cost of the EDFA also grows expensive. In addition, there is a problem of loss in the signal light caused by inserting the modulation elements.
On the other hand, a double-clad EDFA comprises a first clad and a second clad. The first clad surrounds a core doped with a laser medium and functions as a coupler for excitation light, and the second clad is formed around the first clad to provide a waveguide to the first clad. As the first clad has a multi-mode and a relatively large cross-sectional area, it can efficiently couple high-power semiconductor lasers that have multi-mode oscillations, improving the power transmission of the excitation light, and thereby increases the pulse repetition frequency and the average output power. However, in a double-clad EDFA, the excitation efficiency per unit length is lower than a single-clad EDFA, which feeds excitation light directly into the core. Because of this, it is difficult to reduce the length of the fiber in a double-clad EDFA, so there is a problem of an increased loss in the signal light, which is caused by nonlinear effects such as a parametric process or a simulated Raman scattering, occurring in the fiber. As a result, a double-clad EDFA cannot achieve as high a peak power as a single-clad EDFA.